1. Field of the Invention
The present invention relates to a flow verification system and a flow verification method of examining the flow characteristics of a flow controller installed in a gas piping system used in, for example, semiconductor manufacturing equipment.
2. Description of Related Art
For example, film deposition and dry etching apparatuses of the semiconductor manufacturing equipment use so-called special material gases such as silane and phosphine, corrosive gases such as chlorine gas and highly flammable gases such as hydrogen gas. Flow rates of these gases are very strictly controlled for the following reasons: their flow rates directly have an influence on whether a process is successful or not, cost for removing a device installed in the exhaust system is involved, and the gases themselves are expensive, and so on. Since the quantity of gas actually used in the process is at most 500 sccm or so, a known mass flow controller is installed in the piping so as to optimize the flow rate depending on the type of gas and process recipe. In the mass flow controller, flow rates are set by adjusting the applied voltage.
Among process gases, deposition material gases may cause precipitation of solid substances due to their properties, resulting in a change in a flow capacity. Particularly, the mass flow controller is more likely to cause precipitation of solid substances in its inner small tubes than other components and if such precipitation should occur, the flow capacity would be seriously affected. A change in the flow capacity will inevitably change the relationship between the applied voltage and actual flow rate and in that case, even when the flow rate setting remains unchanged, the actual flow rate will change, leading to deterioration in process stability. If the flow capacity has actually changed, a preset value of voltage to be applied must be modified in order to keep the gas flow rate adequate. For this reason, the flow rate of the mass flow controller must be verified.
The flow rate of the mass flow controller is basically verified using a film flow meter. However, in this method, some part of the piping must be removed and after measurement work, it is replaced as it was and checked for leakage. This procedure is troublesome. As one solution to this, Japanese Patent No. 3367811 proposes a method in which the flow rate is verified without removing any pipes.
FIG. 5 shows a configuration of a gas system 100 to which a flow verification system 110 as described in Japanese Patent No. 3367811 is applied.
In the gas system 100, gas lines 101A and 101B converge into a gas supply line 102, which is connected with a processing tank 103. The flow verification system 110 verifies a flow rate of process gases A and B under the control of mass flow controllers 105A and 105B, based on pressure measured by a pressure sensor 108.
In the gas lines 101A and 101B, first cutoff valves 104A and 104B, the mass flow controllers 105A and 105B, and second cutoff valves 106A and 106B are provided in order from upstream respectively. A final cutoff valve 107 is provided in the gas supply line 102 and the pressure sensor 108 and a vent line 109 are located upstream from the final cutoff valve 107. The pressure sensor 108 is connected with the flow verification system 110 to issue a pressure detection signal. Opening and closing of the final cutoff valve 107 are controlled by the flow verification system 110 or a host device 111 connected with the flow verification system 110.
The flow verification system 110 specifies an initial rate of pressure variation for each of the mass flow controllers 105A and 105B immediately after assembling a piping system or replacing the mass flow controllers, as follows.
For example, in specifying an initial rate of pressure variation for the mass flow controller 105A, the flow verification system 110 first opens the first cutoff valve 104A and the second cutoff valve 106A of the gas line 101A and the final cutoff valve 107 of the gas supply line 102, with the second cutoff valve 106B of the gas line 101B closed. The pressure downstream from the mass flow controller 105A is decreased using a vacuum pump or similar device (not shown) connected with the processing tank 103.
After that, the final cutoff valve 107 is closed to cut off the flow of exhaust air to the processing tank 103. At this time, the first and second cutoff valves 104A and 106A are opened and thus the flow rate of the process gas A is controlled by the mass flow controller 105A and the gas A is introduced into a line portion between the mass flow controller 105A and the final cutoff valve 107. Consequently, pressure measured by the pressure sensor 108 gradually increases. The flow verification system 110 samples the pressure measured by the pressure sensor 108 at regular time intervals and calculates a gradient in the pressure variation range with the good linearity by the least square method. The flow verification system 110 stores this gradient as an initial value.
If the process gas A is used for flow verification, the flow verification system 110 samples the pressure measured by the pressure sensor 108 at the regular time intervals with the same procedure as above and calculates the gradient in the pressure variation range with the good linearity by the least square method. The calculated gradient is compared with the initial value. If the calculated gradient does not indicate a change from the initial value, the flow verification system 110 determines that the flow characteristics of the mass flow controller 105A are unchanged (normal). On the contrary, if the calculated gradient indicates a change from the initial value, it determines that the flow characteristics of the mass flow controller 105A have changed and the mass flow controller 105A is out of order.
However, in the conventional flow verification system 110, just after the mass flow controller 105A starts flow control of the process gas A, the pressure unstably varies depending on the flow rate. Therefore, the conventional flow verification system 110 waits a few seconds after the mass flow controller 105A starts the flow control and it is only after the flow is stabilized that it can calculate the gradient in the pressure variation range with the good linearity for the flow verification.
In an actual film deposition process, as soon as the first cutoff valve 104A is opened and the process gas A is introduced into the processing tank 103, a film deposition cycle is started. If one deposition cycle requires 5 to 6 seconds, the process gas A supplied to a wafer initially (for example, one second after start of the flow control by the mass flow controller 105A) largely affects the deposited film quality. For this reason, there has been strong demand for a system which starts flow verification just after the mass flow controller 105A starts flow control, but the conventional flow verification system has not satisfied this demand.